The present invention concerns composite, flexible, resilient materials which contain electrically conductive particles, and more specifically, composite materials which behave as electrical insulators when the pressure on them is less than a certain amount and which become conductive when the pressure is increased above this level. Even more specifically, the invention concerns materials of this type which require no chemical treatment of their constituents. The invention also concerns various industrial applications of the materials.
Flexible and resilient materials which are electrically conductive even in the absence of pressure on their surfaces have been known for a long time. If the surface of such a material is touched lightly with the electrodes of an Ohmmeter, the material is conductive as if it were a piece of metal. These materials consist of a low viscosity polymer (several thousand centipoises only) which is charged with spherical metal particles between 2.5 and 2500 microns in diameter. In general, these materials are made by using a metal powder of definite particle diameter such that the particles are maintained in contact with each other. In order for the particles to come into contact, the viscosity of the binder must be low enough or must become low enough, that the particles can sink towards the bottom of the binder when it is in its liquid state. In order for the entire mass of the composite material to be conductive the binder must be heavily charged with metallic particles. It is general practice to use particles whose surface is formed of a precious metal. The oxide layers which form on particles of common metals, because of corrosive solvents in the polymer, and the elevated cure temperatures prevent the particle to particle contact which is necessary for the material to be electrically conductive even when no pressure is applied to its surface.
If particles of a common metal are used, the material obtained is an insulator when no pressure is applied and becomes conductive only under pressure. When pressure is applied to the surface of the material, the oxide layers on the particles are broken and the material becomes conductive. However, a pressure sensitive material obtained in this way has several limitations. Such a material is not electrically stable because of the variations in the oxide layers and the resultant variations in the pressure required to render the material conductive. In general, the pressure required to render such a material conductive is quite high.
It is known that it is difficult to use metal particles having a form other than spherical in a low viscosity polymer to fabricate materials which are conductive even in the absence of pressure on their surfaces. For example, a low viscosity polymer charged with flake shaped particles is not electrically stable and is not, generally, electrically conductive in the absence of pressure on its surface. An exception to this is a low viscosity polymer which is subjected to strong thermal contraction during cure. However, this requires elevated temperatures.
There are conductive plastics which are resilient and flexible, and which behave as electrical insulators under low pressures but which become electrically conductive if the pressure is increased. However, these materials, even though they are electrically stable, use particles and binders which are chemically treated. These treatments produce intermediate semiconductor regions between the metallic particles and the binder. The use of chemical treatments is a disadvantage because it requires additional production processes which are costly. Another disadvantage of these materials is that the electrical behavior under pressure depends on the chemical processes during fabrication. Therefore, the entire mass of material has the same electrical behavior. There is no possibility of selective modification of various areas of the material after vulcanisation.
Until now, the preference has been to use materials which are electrically conductive even in the absence of pressure on their surface, rather than pressure sensitive materials, because of the problems of instability of the latter. Nevertheless, even though there are applications where it is desirable that the material be conductive even without pressing on its surface, this is most often a disadvantage. In the fabrication of keyboards and switches, for example, it is necessary to prevent the conductive material from creating a short circuit with the metallic contact points of the substrate except at those moments when pressure is applied to the key. This requires additional mechanical parts. Such a material is disadvantageous also when used as connectors of interconnection of electronic components or for devices designed to establish electrical contact with the output pins of circuits. Each piece of conductive material must be electrically isolated from all the others to prevent short circuits between them. There has been a need for a long time of flexible, resilient, materials which become conductive only under pressure; whose fabrication is simple; which exhibit stable electrical behavior; and whose characteristics can easily be modified over a wide range.
Paste-like adhesives which are electrically conductive when applied as films a few mils thick have also been known for a long time. These materials also use precious metal particles. Until now these materials have been used as adhesives to adhere conductive materials to substrates.